The radio terminal is supported with a DRX functionality (DRX: Discontinuous Reception) in 3 GPP LTE (Long Term Evolution), being one of next-generation cellar systems, so as to reduce power consumption of the radio terminal (Non-patent literatures 1 and 2). In the LTE, a period called a DRX cycle that consists of a reception period (On-Duration) and a non-reception period subsequent hereto (Opportunity for DRX) is defined, and repeating these periods allows the DRX to be realized.
The radio terminal shall receive a downlink control channel (PDCCH: Physical Downlink Control Channel) at any time in the On-Duration, and does not need to receive it in the Opportunity for DRX. Additionally, when the radio terminal fails in receiving data during the On-Duration and yet the above data is retransmitted after the On-Duration period, it extends the period in which the PDCCH is received.
Herein, the period in which the radio terminal under DRX operation receives the PDCCH is called Active Time, and the On-Duration is a minimum value of the Active Time. In addition, two DRX states (levels), i.e. “ShortDRX” and “LongDRX” each having a different length of the Opportunity for DRX can be set for each radio terminal. When the radio terminal in a ShortDRX state does not receive the data for a constant period, the LTE takes a DRX state control for transiting to a LongDRX state. Further, a timer (drxShortCycleTimer) is used for determining a state transition from the ShortDRX to the LongDRX. This makes it possible to set the DRX state (level) suitable for a data reception frequency of the radio terminal, and to realize a reduction of power consumption of the radio terminal.
In addition, as the cellar system having the LTE sophisticated therein, LTE-Advanced is standardized. As one of the LTE-Advanced Functionalities, there exists carrier aggregation (Carrier Aggregation: CA) for performing data transmission/reception by simultaneously using a plurality of the component carriers (Component Carrier: CC) for one radio terminal, which serves as a functionality of enhancing a peak date rate for each radio terminal (Non-patent literature 3). Herein the so-called CC is a basic frequency block necessary for realizing the communication between the radio base station and the radio terminal in the LTE. When the CA is carried out, one transport block (a unit for transferring data from an MAC layer to a PHY layer) is transmitted/received on one CC, and a signal process is independently performed for each CC. Additionally, when HARQ is carried out because retransmission of the data is required, the CC used for the first transmission is identical to the CC used for the retransmission.
At present, in 3GPP Standardization, a discussion about the DRX of the radio terminal at the time of the CA is underway, and a method of carrying out identical DRX configuration (setting of a DRX parameter) to all the CCs for performing the Carrier Aggregation (CA) is being investigated. As an actual method of the DRX control (an Active Time control and a DRX state control), a method (A) of cooperatively taking a control among the CCs for performing the CA (Non-patent literature 5) and a method (B) of independently taking a control on each CC (Non-patent literature 6) are proposed. In the method (A) of cooperatively taking a control among the CCs, the DRX state control is commonly taken among the CCs by aligning the Active Time on all the CCs to that of the CC on which the data has been received to the end. On the other hand, in the method (B) of independently taking a control on each CC, the Active Time of each CC is decided based on a data reception situation on each CC, and the DRX state control is also taken independently on each CC.
An example of the method (A) of cooperatively taking a control among the CCs for performing the CA will be explained by employing FIG. 24.
This figure shows a situation in which a certain radio terminal is assigned CC1 to CC3 as the CC for performing the CA and the radio terminal is ready for reception of DL data on all these CCs. Further, a DRX parameter on each CC is identical, and starts of respective DRX cycles are synchronized with each other among the CCs. Upon paying attention to a first-place DRX cycle of the CC1, the Active Time is extended so that the retransmission data can be received because the DL data was not able to be successfully decoded notwithstanding the reception of the DL data in the On-Duration. And, the CC1 transits to the non-reception period (Opportunity for DRX) in which the PDCCH does not need to be received when the retransmission data can be successfully decoded.
Next, upon paying attention to the operations on the CC2 and the CC3 at the DRX cycle having the identical timing, no data is received on the CC2, and the data is received on the CC3 similarly to the CC1. At this time, upon viewing the CC2 and the CC3 separately, there is no necessity for extending the Active Time beyond the On-Duration on the CC2, and the Active Time is extended on the CC3; however the Active Time of the CC3 may be shorter than that of the CC1.
However, in the method (A), all the Active Times of the CC1 to CC3 are ones depicted by dotted lines in the figure because the Active Times of the CC2 and the CC3 are decided so as to match with that of the CC1 requiring the longest Active Time. Herein, the hatching portion in the figure is the Active Time that has been originally unnecessarily extended for the above CC. The situation is similar with the second-place DRX cycle and the third-place DRX cycle as well, the Active Time is controlled so as to match with that of the CC3 at the second-place DRX cycle and with the CC2 at third-place DRX cycle, respectively.
Next, an example of the method (B) of independently taking a control on each CC will be explained by employing FIG. 25.
Similarly to FIG. 24, the radio terminal is ready for reception of the DL data on the CC1 to CC3, the DRX parameter is identical on each CC, and the starts of respective DRX cycles are synchronized with each other among the CCs. In addition, it is assumed that the radio terminal is firstly in a state of the ShortDRX, and transits to the LongDRX when the data is not received over three-time ShortDRXs (the length of drxShortCycleTimer is equivalent to three times that of the ShortDRX cycle).
Upon paying attention to the first-place DRX cycle, the drxShortCycleTimers are independently started on all the CCs. The data is received on the CC1 and the CC3, and the drxShortCycleTimer is restarted after the data reception is completed (it is again started from the initial value). The data is not received on the CC2, whereby the timer is successively running as is. In such a manner, the drxShortCycleTimer is activated on each CC. The CC3, on which the drxShortCycleTimer expires at the fifth-place DRX cycle earliest, transits to the LongDRX, and the CC1 transits to the LongDRX at the sixth-place DRX cycle. On the other hand, the CC2 receives the data also at the sixth-place DRX cycle with still the state of the ShortDRX.
As a result, only the CC2 can be used at the seventh-place DRX cycle and the eighth-place DRX cycle, and the CC1 and the CC3 cannot be used because they are in the non-reception period of the LongDRX. Additionally, it is at a timing of the On-Duration next to the LongDRX cycle that the CC1 and the CC3 can be used again.